Global Warming: Lessons Taught By Snails And Crabs

As acting director of Stanford's Hopkins Marine Station, Somero has to walk only a few dozen steps from his lab to the waters of Monterey Bay, where he and other marine biologists have found disturbing signs that higher ocean temperatures have transformed wildlife populations in the Pacific.

"The effects of global warming already seem evident," says Somero, the David and Lucile Packard Professor in Marine Science at Stanford.

"The vast majority of scientists regard continued warming of the Earth as inevitable," he adds, "and some of the best information we have on the potential effects of climate change comes from data collected right here at Hopkins."

Somero points to a three-year study conducted in the 1930s, when Stanford graduate student Willis Hewatt counted and identified all of the marine invertebrates living in a 95-square-yard section of intertidal shoreline near the marine lab located in Pacific Grove, Calif.

The study was all but forgotten until 1993, when researchers decided to re-survey the same area to determine if the types of invertebrate species present at Hopkins had shifted during the past few decades. Scientists counted everything from limpets to crabs and discovered that marine populations had changed dramatically in just 60 years.

"There was a significant decrease in northern species -- those that tend to occur to the north of Monterey Bay, but eight out of nine southern species increased in abundance," says Somero.

"The overall message in these data," he notes, "is that cold-loving species tended to move out, and warm-loving species moved in."

Could this shift in species distribution have been caused by a change in climate?

To answer that, researchers needed to determine if the temperature of Monterey Bay had changed since the 1930s.

Fortunately, notes Somero, Hopkins Marine Station personnel have been meticulously recording seawater temperatures every day for nearly 80 years, and a review of those records showed that, indeed, Monterey Bay had gotten warmer.

"The data showed that, during the 60-year interval between the two animal surveys, annual mean water temperatures increased on average by about 1.3 F [0.7 C]," says Somero.

More significantly, he adds, peak summer temperatures in August rose nearly 4 F (2.2 C).

Although these temperature increases seem relatively small, Somero believes they may have been substantial enough to push some species over the edge of what he calls their thermal tolerance range.

"When thermal stress pushes body temperatures to values that are unnaturally high or low, biochemical structures and the physiological processes they support -- such as the heart and nervous system -- --may be severely, and perhaps lethally, upset," Somero observes.

Climatologists predict that, if global warming continues at its current pace, the average temperature of the Earth could increase another a 6 F (3.3 C) in the next 50 years.

What effect will these rising temperatures have on marine organisms --- especially on vulnerable intertidal creatures that frequently are exposed to the hot rays of the sun during low tide?

To find out, Somero, postdoctoral scholar Lars Tomanek and former graduate student Jonathan Stillman (now at Occidental College) decided to investigate thermal tolerance limits in two groups of common Pacific invertebrates -- porcelain crabs (genus Petrolisthes) and snails (genus Tegula).

The researchers wanted to see if intertidal crabs and snails are more susceptible to heat than their subtidal cousins, which spend their entire lives under water.

Lessons from crabs

Somero and Stillman collected 20 species of porcelain crabs from intertidal and subtidal habitats in four Pacific regions: temperate coastal waters off California and Chile, and subtropical and tropical areas off Mexico and Panama.

The thermal tolerance limit of each species was determined by raising the water temperature in an experimental chamber by 1.8 F (1 C) every 15 minutes, then examining the number of survivors at each temperature interval.

"The rate of 1 C per 15 minutes reflects the temperature increase that porcelain crabs experience during extremely hot low tide periods," Somero explains.

The results, recently published in Physiological and Biochemical Zoology, showed that porcelain crab species living at the surface appear more vulnerable to global warming than those that are always submerged in subtidal habitats.

For example, intertidal species from tropical waters off Mexico and Panama succumbed when the thermometer reached 107 F (41 C) -- only about 1.8 F (1 C) higher than the maximum temperature they currently experience in the wild.

Similar results were found among intertidal crabs from cooler waters off California and Chile. These animals could tolerate temperatures between 90 to 95 F (32 to 35 C) -- only slightly higher than their maximum habitat temperature of 88 F (31 C).

One intertidal crab species included in the study, Petrolisthes cinctipes, was a common inhabitant of Monterey Bay 60 years ago, according to the ocean survey conducted at Hopkins in the 1930s.

"But P. cinctipes showed a significant decline in the 1993 re-survey, a finding that is consistent with our physiological data," observes Somero.

Overall, he says, species from intertidal locations around the Pacific already are living at the edge of their thermal limits and might not be able to survive even slight temperature increases.

In contrast, subtidal species from all habitats turned out to have thermal tolerance limits that, while lower than intertidal species, were much higher than the maximum water temperatures they encounter in nature.

Lessons from snails

In another study published in Physiological and Biochemical Zoology, Somero and Tomanek looked at the importance of temperature in determining distribution patterns of another group of marine organisms, snails of the genus Tegula.

They wanted to find out if snails continue to manufacture proteins in their cells when exposed to high temperatures.

Somero and Tomanek compared two snail species commonly found in Monterey Bay. One type, Tegula funebralis, lives near the surface and frequently is exposed to full sun during low tide. The other, T. brunnea, usually is submerged and therefore experiences less intense heat during the day.

When kept at a temperature of 86 F (30 C) for 2.5 hours, T. funebralis continued to manufacture proteins -- unlike its cousin, T. brunnea, which virtually stopped all protein production and eventually died.

"These data help to explain the different vertical distribution of these two species of Tegula," notes Somero. "The lower-occurring species, T. brunnea, simply cannot continue to manufacture proteins at temperatures routinely experienced by its higher-occurring cousin, T. funebralis.

"But even T. funebralis is poised near its thermal tolerance limit for protein synthesis," he concludes, "and if an organism can no longer make proteins --- the molecules responsible for metabolism -- then its survival is in jeopardy. Therefore, additional warming could create serious problems for both snail species."

Ever northward

These experiments may help to explain why so many marine invertebrates, once common in Monterey Bay, have become less abundant there.

"It appears that some species are at the upper boundary of their thermal tolerance range," says Somero, "so higher ocean temperatures may force them to seek out cooler pastures."

And even if these displaced species avoid extinction by moving to cooler regions, their disappearance could significantly perturb the ecosystem, he maintains.

"If snails, crabs and additional prey items disappear, then other species dependent on these food sources may encounter serious nutritional problems, even if they can tolerate elevated temperatures," Somero adds.

But ocean critters are not the only ones affected by climate change. Somero points to studies showing that a wide variety of plants, insects, birds and other organisms are expanding their ranges northward or moving to higher elevations -- a likely result of global warming.

If the trend continues, warn scientists, it could have serious consequences for human health. A 1998 survey concluded that malaria, dengue fever and other mosquito-borne diseases are now occurring in the highlands of Asia, Central Africa and Latin America -- an indication that mosquitoes are moving into habitats once considered too cold for their survival.

The World Wide Fund for Nature recently forecast that 20 percent of all organisms now living in colder climates could be wiped out by the end of the century. The WWF report predicts that, as global warming accelerates, plants and animals will invade cooler habitats, but many will be unable to move fast enough to avoid extinction.

Whether this catastrophic scenario actually occurs is a matter of debate, but according to Somero, the ability of an organism to quickly adapt to temperature changes may be the key to its survival.

For example, the snail species T. funebralis might be able to handle the predicted 6 F (3.3 C) rise in ocean temperature, but its cousin T. brunnea may have to undergo a biochemical change that would give its cells the ability to manufacture proteins in warmer conditions.

"Attempts to increase the thermal limits of protein production by holding snails at warm temperatures in the laboratory led to no upward shift in the temperatures at which proteins could be made," Somero observes.

Therefore, he points out, adapting to rising temperatures may require changes in a species' genes over many generations -- an evolutionary process that may be too slow to keep pace with global warming.

"Even though organisms have been evolving and adapting to a changing environment since the very origin of life," adds Somero, "it is by no means clear that the pace and magnitude of the change now occurring can be dealt with by many species.

"The rates at which they can adapt are finite and may be much slower than the rate at which the climate is shifting," he concludes.

"Our biosphere could be in for some serious problems."